The present invention relates generally to protective circuits for adjustable speed drives for AC electric motors, and in particular to a DC bus clamping circuit providing protection against component failure during an over voltage fault or adverse operating conditions.
Common AC motors use three-phase electrical power connected to the stator windings of a motor to run the motor. Each stator winding is connected to a different conductor from a three-phase power source, in which each conductor delivers a different phase of the electrical power to the motor. The three-phase power source may be a direct connection to line power, but more commonly, the motor is connected to an adjustable speed motor drive (ASD). The ASD allows for speed control of the motor not available by connecting the motor directly to line power.
As is known in the art, there are many electrical topologies for ASDs used to convert the fixed voltage and frequency from the line input into a controlled voltage and frequency output for a three-phase motor. One common topology includes a rectifier section which converts the line power into a DC voltage used to charge a DC bus section of the ASD. An inverter section then uses a switching algorithm, typically pulse width modulation (PWM), to convert the DC voltage from the DC bus into a variable voltage and frequency output to the motor. Controlling the variable voltage and frequency output to the motor controls the speed at which the motor rotates.
While an ASD provides the benefit of speed control for a motor, it can also introduce negative side effects. PWM algorithms utilize high frequency switching to alternate between short pulses of DC voltage from the DC Bus and no voltage being output to the motor. The duration of these pulses vary such that averaging the DC voltage of the pulse along with the periods during which no voltage is being output results in a lower fundamental AC voltage and frequency seen at the motor, providing a three phase AC waveform suitable for speed control of a motor. However, the high frequency voltage pulses also introduce high frequency electrical transients at the motor output. The motor is connected to the ASD by a power cable. The power cable may introduce a common mode capacitance to the system. In systems where a high capacitance may be present, such as with long power cables, multiple motors operating from a common DC bus, or a high horsepower motor, the high capacitance output creates an impedance that interacts with the high frequency voltage pulses to establish reflected voltage and current waveforms at the output. These reflected waveforms produce voltage and current surges that can create an over voltage condition for both the motor and the drive components. If the over voltage condition is generated in the motor windings, the life of the motor may be shortened. If the over voltage condition occurs at the drive, the voltage seen between the DC bus and ground may be much higher than normal, creating a voltage stress on the insulation of the drive components (e.g. switching mode power supply transformer, IGBT gate opto-coupler, DC link chokes, and common mode capacitors). This voltage stress may similarly result in a shortened life of the drive components.
The inventors have identified a second condition in which an overvoltage condition for the drive may be generated. If multiple motors are operating from a common DC bus, it is possible that a single motor may fail while the other motors continue operation. It is also possible that a single motor may be damaged yet continue to operate. For example, if one winding in a motor has a ground fault condition, it may draw substantial currents while that phase is switched on by the inverter and substantial voltage or current spikes can be generated. Because each of the multiple motors operates from a common DC bus, the other motors may similarly be affected by the voltage or current spikes. Further, the multiple inverter sections may allow for conductive paths not present in single inverter systems that allow for voltage or current surges that can create an over voltage condition. Therefore, if a single motor fails, or begins to fail, it may affect other motors or drives on the common DC bus and cause subsequent motor or drive failures.
Prior attempts to protect against over voltage conditions include detecting rising voltage levels on the DC bus and shutting off the inverter section in response to the over voltage condition. This approach may result in undesirable down time of a motor or even an entire process line on which the motor is installed.
Other attempts to protect against over voltage conditions have included snubber circuits at the motor terminals or across the DC bus. Such circuits have been designed to handle continuous operation and are, therefore, required to handle significant voltages and currents. This requirement results in high voltage and power ratings for the components or alternatively multiple components designed to share the load. The increased power ratings or number of components results in an increased cost of the circuit.